The present invention relates to the synthesis of hydrogen peroxide, and more particularly, to the synthesis of hydrogen peroxide without the use of an organic solvent.
Hydrogen peroxide (H2O2) is often considered to be a xe2x80x9cgreenxe2x80x9d material, in that it is increasingly used to replace chlorine-containing reagents in paper bleaching and in water purification. For this reason, as well as others, hydrogen peroxide production is estimated to increase steadily through the beginning of the next century.
The production of hydrogen peroxide is a mature process in that the general procedure has not changed appreciably in twenty years. Indeed, recent research publications in the area of hydrogen peroxide synthesis are somewhat scarce. Typically, hydrogen peroxide is generated in a two-step process, wherein hydrogen is first reacted with a 2-alkyl anthraquinone (usually 2-ethyl or 2-amyl anthraquinone) in an organic solvent to produce the corresponding tetrahydroquinone (2-alkyl tetrahydroquinone). The reaction is catalyzed by a simple palladium-on-alumina catalyst. Conditions for this reaction are typically 30 to 70xc2x0 C. with hydrogen pressures up to 300 psi. Given the nature of the reactants, the reactor contains three phases (gas, liquid, and solid catalyst) and previous work has shown that the reaction is completely mass transfer limited, such that the rate of the reaction is essentially the rate at which hydrogen diffuses into the liquid phase. Partly as a result of this inefficiency of hydrogen use, side reactions (hydrogenation of one or both of the aromatic rings) also occur, and byproducts build up during repeated cycling of the anthraquinone. These byproducts must periodically be removed and treated. The organic solvent employed is typically a mixture of an aromatic (a good solvent for the anthraquinone) and a long-chain alcohol (a good solvent for the hydroquinone).
The second step of the process involves oxidation of the hydroquinone, regenerating the anthraquinone and producing hydrogen peroxide. Here the catalyst is retained in the first reactor, and the solution of alkyl anthraquinone, alkyl tetrahydroquinone and organic solvent (the working solution) is transferred to the second reactor, where the hydroquinone is reacted with oxygen (as air or oxygen). This reaction is uncatalyzed. Similar to the first reaction, the second reaction is mass transfer limited by the rate at which oxygen can diffuse from the gas to liquid phases. Finally, the hydrogen peroxide is stripped from the organic solvent via liquid-liquid extraction with water and sold as an aqueous mixture (usually 30 to 50%).
Because the final step in the production of hydrogen peroxide involves a liquid-liquid extraction between aqueous and organic phases, the final product is contaminated to some extent by the organic phase. Given that H2O2 is promoted as a green reagent for paper production, and is also used in water purification, the organics in the final product must be minimized. Significant effort is thus made to strip the organic contaminants from the product.
It is, therefore, very desirable to develop reactants and processes for the synthesis of hydrogen peroxide that minimize or eliminate the use of organic solvents.
In general, the present invention provides a method for synthesizing hydrogen peroxide, comprising the steps of:
synthesizing an analog of anthraquinone that is miscible with (in the case of a liquid analog) or soluble in (in the case of a solid analog) carbon dioxide;
reacting the analog of anthraquinone with hydrogen in carbon dioxide to produce a corresponding analog of tetrahydroquinone; and
reacting the analog of tetrahydroquinone with oxygen to produce the hydrogen peroxide and regenerate the analog of anthraquinone.
Preferably, the regenerated analog of anthraquinone is recycled for future use.
The step of synthesizing an analog of anthraquinone that is miscible in carbon dioxide preferably comprises the step of attaching to anthraquinone at least one modifying or functional group that is relatively highly soluble in CO2 (xe2x80x9cCO2-philicxe2x80x9d). The miscibility/solubility of the resulting analogs of anthraquinone are several orders of magnitude greater at the operating pressures of the present invention than the solubility of 2-alkyl anthraquinone in carbon dioxide at pressures equal to or below 5000 psi. Alkyl-anthraquinones used in the commercial synthesis of hydrogen peroxide do not exhibit appreciable solubility in carbon dioxide at pressures below 5000 psi. In that regard, a number of studies have explored the solubility of alkyl-functional anthraquinones in carbon dioxide and found generally that the system exhibits solid-fluid phase behavior with maximum solubilities of approximately 10xe2x88x922 mM. See, for example, Joung, S. N., Yoo, K. P., J. Chem. Eng. Data, 43, 9 (1998). Coutsikos, P., Magoulos, K., Tassios, D., J. Chem. Eng. Data, 42, 463 (1997). Swidersky, P., Tuma, D., Schneider, G. M., J., Supercrit. Fl., 9, 12 (1996). ibid, 8, 100 (1995).
A liquid-liquid phase envelope is preferably formed in the functionalized anthraquinone-carbon dioxide systems of the present invention at relatively moderate pressures. The operating pressure at which the analogs of anthraquinone (and preferably the analogs of hydroquinone) are reacted in carbon dioxide is preferably no greater than approximately 5000 psi. More preferably, the operating pressure is no greater than approximately 3000 psi. Even more preferably, the operating pressure is no greater than approximately 2500 psi. Most preferably, the operating pressure is no greater than approximately 1500 psi. The operating pressure at which the analogs of anthraquinone are reacted with hydrogen (and, preferably, the operating pressure at which the analogs of hydroquinone are reacted with oxygen) is preferably chosen such that it is above the cloud point curve (and, preferably, above the maximum of the cloud point curve) in the liquid-liquid phase envelop (or liquid-fluid phase envelope when operating at supercritical conditions). In the region above the cloud point curve, single-phase behavior is observed.
The operating temperature of the present reactions is preferably between approximately 0xc2x0 C. and approximately 100xc2x0 C. The operating temperature of the present reactions is more preferably between approximately 20xc2x0 C. and approximately 40xc2x0 C. Most preferably, the operating temperature of the present reactions is approximately 25xc2x0 C. (room temperature).
Preferably, the CO2-philic functionalized anthraquinones and the corresponding hydroquinones of the present invention exhibit reactivity similar to the 2-alkyl anthraquinone and hydraquinones used in the current commercial synthesis of hydrogen peroxide. Indeed, the kinetic rate constants calculated for the oxygenation of the functionalized anthraquinones of the present invention were found to be approximately ten time greater than anthraqinone. The use of CO2-philic groups to increase the solubility of a molecule in carbon dioxide is also discussed in U.S. Pat. No. 5,641,887, the disclosure of which is incorporated herein by reference.
In general, the analog of anthraquinone preferably has the formula: 
At least one of R1, R2, R3, R4, R5, R6, R7, and R8 (corresponding to the 1, 2, 3, 4, 5, 6, 7, and 8 carbons on the anthraquinone ring structure) is a modifying group or functional group that is miscible/souble in carbon dioxide. Attachment of one or more such CO2-philic groups to anthraquinone results in an analog of anthraquinone that is miscible/soluble in carbon dioxide. In that regard, R1, R2, R3, R4, R5, R6, R7, and R8 are preferably, independently, the same or different, H, RC or RSRC, wherein RS is a connector or a spacer group and RC is a fluoroalkyl (fluorinated alkyl) group, a fluoroether (fluorinated ether) group, a silicone group, an alkylene oxide group, a phosphazene group or a fluorinated acrylate group. At least one of R1, R2, R3, R4, R5, R6, R7, and R8 is not H. Preferably, RC is a fluoroalkyl group, a fluoroether group or an alkylene oxide group. More preferably, RC is a fluoroether group or an alkylene oxide group.
The spacer group, RS, when present, can simply be a connective group used to attach a CO2-philic group to anthraquinone or can additionally act to space the CO2-philic group away from the anthraquinone. The spacer group is preferably a group which provides a simple synthetic route to achieve the desired analog of anthraquinone without substantially adversely affecting the miscibility of the analog of anthraquinone in carbon dioxide or the reactivity of the analog of anthraquinone and the corresponding hydraquinone in the synthesis of hydrogen peroxide. For example, the spacer group can be an alkylene group, an amino group, an amido group, an ester group or an alkyl ester group. As used herein in connection with RS, the term xe2x80x9calkylene groupxe2x80x9d refers to a linear or branched alkylene group. A linear alkylene group, for example, has the formula xe2x80x94(CH2)nxe2x80x94. As used herein in connection with RS, the term xe2x80x9camino groupxe2x80x9d refers to a secondary amino group having the formula xe2x80x94NHxe2x80x94 or a tertiary amino group having the formula xe2x80x94NR11Hxe2x80x94, wherein R11 can generally be any substituent that doesn""t interfere with the reactivity of the desired analog. For example, R11 can be an alkyl group. As used herein in connection with RS, the term xe2x80x9camido groupxe2x80x9d refers to secondary amido having the formula xe2x80x94NHCOxe2x80x94, or a tertiary amido group having the formula xe2x80x94NR11COxe2x80x94 wherein R11 is as defined above. As used herein in connection with RS, the term xe2x80x9cester groupxe2x80x9d refers to a group having the formula xe2x80x94OCOxe2x80x94. As used herein in connection with RS, the term xe2x80x9calkyl ester groupxe2x80x9d refers to a group having the formula xe2x80x94R12OCOxe2x80x94, wherein R12 is an alkyl group. The spacer group itself need not be CO2-philic. If it is desired to use the spacer group to space the CO2-philic group away from the anthraquinone ring structure, an alkalene group is preferably used, either alone or in combination with another connective group.
The total molecular weight of the CO2-philic groups RC attached to the analog of anthraquinone is preferably between approximately 200 and approximately 7500 to make the analog of anthraquinone miscible/soluble in carbon dioxide. One or more CO2-philic groups can be attached to the anthraquinone ring structure. For example, each of R2, R3, R6, and R7, can comprise a perfluoroalkyl group having a molecular weight of 50. More preferably, the total molecular weight of the CO2-philic groups is between approximately 500 and approximately 5000. Most preferably, the total molecular weight of the CO2-philic groups is between approximately 500 and approximately 1500.
The fluoroalkyl groups of the present invention are preferably linear perfluoroalkyl groups having the formula/repeat group:
xe2x80x94(CF2)gxe2x80x94.
wherein g is an integer.
The fluoroether groups of the present invention are preferably perfluorinated and have the formula/repeat group: 
wherein each of x, y and z is an integer greater than or equal to 0 and at least one of x, y and z is not equal to 0.
The silicone groups of the present invention preferably have the formula/repeat group(s): 
wherein R9 and R10 are chosen to not substantially affect the CO2-philic nature of the silicone group or the reactivity of the functionalized analogs of anthraquinone. R9 and R10 may, for example, be, independently, the same or different, H, an alkyl group, an aryl group, an alkenyl group, or an alkoxyl group, and wherein b is an integer. Preferably, R9 and/or R10 is a fluoroalkyl group.
The alkylene oxide groups of the present invention preferably have the formula/repeat group: 
wherein d is an integer and e is an integer.
The fluorinated acrylate groups of the present invention preferably have the formula/repeat group: 
wherein g and j are integers.
The phosphazine groups of the present invention preferably have the formula/repeat group: 
wherein m is an integer and R9 and R10 are as defined above.
The oxidation of the hydroquinone of the present invention preferably takes place in carbon dioxide at substantially the same pressure as the hydrogenation reaction. The hydrogen peroxide product of the present invention is preferably recovered via a liquid-liquid extraction between the carbon dioxide phase and an aqueous phase. The liquid-liquid extraction is preferably conducted without significantly reducing the operating pressure. Likewise, the carbon dioxide is preferably recycled to the extractor without a significant drop in pressure. Such a process for separation/recovery of hydrogen peroxide product avoids the high costs associated with recompression, while taking full advantage of carbon dioxide""s green properties in running a contamination-free liquid-liquid extraction between a carbon dioxide phase and an aqueous phase.
Moreover, using carbon dioxide as the solvent for the process of the present invention allows one to generate a single phase system of hydrogen plus anthraquinone (for the first reaction of the synthesis), or oxygen plus tetrahydroanthraquinone or tetrahydroquinone (for the second reaction of the synthesis). It is known that hydrogen is completely miscible with carbon dioxide above a temperature of approximately 31xc2x0 C. Hydrogen and carbon dioxide have been found to not form separate phases under the operating conditions of the present invention. The reactions can thus be carried out without the mass transfer limitation of the current commercial process for the synthesis of hydrogen peroxide, suggesting that one could operate more efficiently, using less hydrogen and/or at lower temperatures, while producing fewer byproducts.
Furthermore, the operating pH for the stripping operation to recover the hydrogen peroxide from the organic phase into the aqueous stream in the current commercial process for the synthesis of hydrogen peroxide is preferably approximately 3.0 to partition the hydrogen peroxide into the aqueous phase. Because the carbon dioxide of the present invention dissolves in water to form carbonic acid, the pH of the water in the presence of high pressure carbon dioxide is approximately 3.0, assisting in partitioning the hydrogen peroxide into the aqueous phase.
The present invention also provides a chemical compound having the formula: 
wherein R1, R2, R3, R4, R5, R6, R7, and R8 are as described above.
Further, the present invention provides a chemical compound having the formula: 
wherein R1, R2, R3, R4, R5, R6, R7, and R8 are as described above.
Still further, the present invention provides method for synthesizing hydrogen peroxide, comprising the steps of:
synthesizing an analog of anthraquinone having the formula: 
wherein R1, R2, R3, R4, R5, R6, R7, and R8 are as described above;
reacting the analog of anthraquinone with hydrogen to produce a corresponding tetrahydroquinone having the formula; 
reacting the tetrahydroquinone with oxygen to produce the hydrogen peroxide and regenerate the analog of anthraquinone.
It has been found experimentally that analogs of anthraquinone functionalized with the CO2-philic functional groups of the present invention are typically liquids at relatively low temperatures and pressures. For example, anthraquinones functionalized with fluoroether groups of the present invention are typically liquid at room temperature and one atmosphere pressure. Unlike the current commercial process using 2-alkyl anthraquinone (a solid), one can generate hydrogen peroxide from the liquid functionalized analogs of anthraquinones of the present invention without the use of any solvent (including carbon dioxide). In such a process, the liquid analog of anthraquinone is reacted with hydrogen in the first reaction to produce the corresponding hydroquinone. The corresponding hydroquinone is then oxidated as described above to produce hydrogen peroxide. The hydrogen peroxide product is preferably recovered via liquid-liquid extraction with an aqueous phase. The CO2-philic groups of the present invention generally reduce the solubility of the analogs of anthraquinone in water, typically rendering the anthraquinone very hydrophobic and greatly reducing contamination of the aqueous phase therewith (as compared to the current commercial process) during extraction of the hydrogen peroxide product. Synthesizing hydrogen peroxide with a liquid analog of anthraquinone and without solvent reduces equipment costs as compared to synthesis in carbon dioxide, but, unlike the process in carbon dioxide, the oxidation and hydrogenation reaction would be mass-transfer limited.